Impact of the Early Eocene Climatic Optimum (EECO; ~53-49 Ma) on planktic foraminiferal assemblage (Pacific Ocean, sites 1209-1210). 

Past warm events offer windows into the biotic response to extreme warmth. The early Eocene interval records the highest global average temperature and CO2 levels of the Cenozoic. Several transient global warming events occur within the Early Eocene Climatic Optimum (EECO, 53-49 Ma), offering an opportunity to investigate the impact of both long term and transient warm climatic conditions on planktic foraminifera. We analyse the planktic foraminiferal record across the EECO obtained from tropical Pacific ODP sites 1209-1210 (Shatsky Rise). These sites have an excellent age model and stable isotope ratios enabling linkage of the biotic data with the climate and carbon cycle spanning the EECO. We combine indicators of carbonate production and preservation [fragmentation index (FI) as a dissolution proxy, weight percent coarse fraction (CF) as foraminiferal production and preservation index, and Foraminiferal Mass Accumulation Rate (FMAR) as foraminiferal production proxy] with changes in planktic foraminiferal assemblages and test-size. At the EECO onset, the abundance of the genus Morozovella (53.28 Ma) and Chiloguembelina (52.85 Ma) decreased at Shatsky Rise sites, confirming previous Atlantic Ocean data and thus pointing towards global decline of these genera. We hypothesise that a reduction in foraminiferal mass accumulation and assemblage test-size would follow the drop in Morozovellids abundance, given their dominance and large size in early Eocene tropical assemblages. In contrast, we record a slight increase in test-size within assemblages and a relatively stable FMAR. These changes may be controlled by growing dominance of the genus Acarinina indicating an ability of this species to benefit from the environmental conditions. In addition, we observe a relatively stable FMAR at decreasing CF which may be linked to either increased carbonate dissolution or enhanced calcareous nannofossil productivity (or a combination of both) reducing foraminiferal relative contribution to the sediment. Even though the pronounced warming during the EECO strongly altered the planktic foraminiferal assemblage composition resulting in the decrease in abundance of some genera, species replacement within communities highlights the resilience of pelagic carbonate production.